I've been playing this off and on since Jeff mentioned it on the bombcast. I only noticed yesterday though, that when entering the dungeon you can choose to go back to earlier difficulties. There are "<<" and ">>" buttons next to your current rank name at the top of the screen that lists your missions. I've gone back to the tutorial a few times, for a laugh, and found out that the godzilla like monster at the end of it is actually quite difficult. Haven't managed to beat it yet, even though I've killed several dragons.
Has anyone else noticed this feature? Were you able to kill godzilla?
I'm currently stuck on treants at my actual rank. But everyone knows they're OP!